Transient absorption (TA) refers to spectral measurements of the absorption of light by a material which has first been "pumped" from its ground electronic state to an excited electronic state by a transient pulse of light (pump beam). A probe pulse of light (probe beam) then follows at some fixed delay time after the pump, and the absorption of the probe beam by the material, as a function of delay time and wavelength, yields a series of TA spectra. These spectra are useful to examine and deduce the structure of the electronic energy levels above the electronic ground state, as well as examining the mechanisms for relaxation of electrons from their excited states back down to the ground state. It is desirable to know these properties of materials, both for purposes of basic understanding, and because the operation of electronic and optical devices made from such materials often depends sensitively on these properties. For example, inorganic semiconductors such as GaAs have been widely studied using TA in order to improve the operation of devices such as diode lasers.
The most common form of prior art TA spectral measurements utilizes a chargecoupled device (CCD) camera and a spectrometer for the rapid acquisition of an entire TA spectrum at a chosen time delay. This measurement methodology is known as single-shot TA spectroscopy. However, the phenomenon known as "chirp" may greatly affect the measurement data collected. In particular, "chirp" refers to the different arrival time of different wavelengths of light within a broadband probe pulse. Chirp can arise from many sources, but the most common source is material dispersion (the term forthe phenomenon that the refractive index of materials changes as a function of wavelength of light). For example, a broadband probe pulse passing through a glass lens or window would be subjected to chirp, because the longer wavelength portions of the pulse travel faster in the material due to the lower index of refraction of glass. Prior art broadband probe pulses "continuum" beams) are generated by focusing picosecond or femtosecond pulses through a transparent material such as water, glass, or sapphire. Hence, the use of the prior art continuum generation process inherently involves creation of chirp. There is no known method for preventing chirp or for completely removing chirp from a broadband continuum (spectral width more than 200 nanometers). As a result of chirp, the delay time for a transient absorption spectrum may not be uniquely defined for every wavelength. In particular, if the delay time is less than the total chirp over the probe pulse spectral range, then the TA spectrum will be distorted by the chirp. For example, in experiments, the chirp is approximately one picosecond across the visible spectrum (450-750 nanometers). Hence, visible TA spectra are severely distorted for delay times less than 1 picosecond.
Prior art single-shot TA methodologies can be deficient for several reasons. First, as discussed above, the chirp phenomena may affect the measurements obtained. The chirp can be eliminated over a narrow spectral range by pulse compression (e.g., by using transform limited pulses). Further, transform limited pulses allow the examined spectra to remain undistorted on all time scales. However, the bandwidth for the shortest obtainable pulses (such as, for example, five femtosecond pulses) is only approximately 200 nanometers, a fraction of what is achievable with continuum generation. Therefore, broadband spectroscopy beyond the transform limit requires chirp correction. Scientists normally attempt to correct the single-shot TA chirp effect by applying numerical correction. Numerical correction requires that uncorrected spectra be taken at closely spaced time intervals and deconvolved based on the measured chirp. In using the single-shot TA methodology, a numerical correction is usually applied because the entire (chirped) spectrum is obtained. Another serious limitation to single-shot acquisition is that array detectors preclude the use of electronic noise suppression in either the frequency (lock-in detection) or the time (gated integration) domains. And, the primary obstacle in using a CCD camera is the low-frequency noise generated by the camera itself, which sets a detection limit for single-shot TA of approximately 0.2%.
The prior art measurement techniques described above are faulty for several other reasons. CCD cameras, at relatively high incident light intensities, are easily saturated by the continuum beam. Since the continuum has a strongly varying intensity with wavelength (see, e.g., FIG. 2), it is difficult to obtain TA spectra over a broad spectral range which do not either saturate the camera or result in poor signal-to-noise ratios. Further, currently available scientific-grade CCD cameras are expensive. Accordingly, as those of skill in the art will appreciate, a simple method and apparatus is desirable for measuring TA spectra which are undistorted by effects of chirp and intensity variations on the continuum probe over a broad spectral range (visible to near-infrared range) which has high sensitivity (on the order of approximately 10.sup.-5).
With the widespread availability of tunable, millijoule pulse energy, kHz repetition-rate femtosecond solid-state Ti:sapphire lasers, femtosecond transient absorption (TA) measuring techniques are now becoming the standard techniques for measuring ultra fast electronic and vibrational processes in material physics, chemistry, and biology. Unlike prior art TA measuring techniques, femtosecond chirp-free TA measuring techniques have the advantage of measuring subpicosecond energy relaxation, which is an important consideration in examining materials because it is during these short delay times that more spectral distortion occurs.
Further, phase-sensitive detection has long been a standard for measuring single wavelength pump-probe dynamics with high-repetition-rate (100-MHZ) lasers. This technique involves modulation of the intensity of the pump beam at a frequency which is lower than the repetition rate f (e.g. by using a mechanical chopper or acousto-optic or electro-optic modulator). A lock-in amplifier is then used to detect the modulation at the same frequency, which is transferred to the probe beam by the TA signal in the material. However, two modifications are necessary to optimize this technique for kilohertz repetition rate sources. First, differential amplification is necessary prior to the lock-in as it allows one to null the background signal due to the kilohertz laser pulse train, which prevents overload of the lock-in owing to insufficient dynamic reserve. The intensity variation over the continuum spectrum (as seen in FIG. 2) also makes the use of differential amplification important. In this fashion, the dynamic range is determined primarily by the photodiodes and is much greater than the range for CCD cameras. Second, synchronous modulation (chopper synchronized to exactly half the repetition rate) is necessary as it prevents phase drift between pump and chopper.
As is known to those of skill in the art, broadband femtosecond continuum pulses can be generated through self-phase modulation by focusing a laserthrough a transparent medium. It is possible to generate tunable probe beams at fixed wavelengths by other means (e.g. frequency-doubling, parametric generation, sum- and difference-frequency generation). However, no means are known other than continuum generation to obtain short pulses of light which simultaneously possess many different wavelengths. As such, a benefit of applying continuum pulses to TA methodologies is that they provide an opportunity to examine the energy structure of materials over a broader spectral width. In addition to the desired broad spectral width, continuum pulses have two unique characteristics uncommon to narrow band light sources. First, a continuum pulse exhibits a frequency temporal dispersion (or, chirp) as the pulse engages the material so that the red portion of the pulse arrives at an earlier time than the blue portion as described above. Second, a continuum exhibits a strong spectral nonuniformity with an intensity spike at the pump wavelength. Graphically, this is illustrated in FIGS. 1 and 2.
Accordingly, it is an object of the present invention to provide a highly sensitive, chirp-free femtosecond transient absorption spectroscopy scanning method which permits direct visualization and modeling of ultra fast, broadband energy relaxation.
It is another object of the present invention to provide a highly sensitive, chirp-free femtosecond transient absorption spectroscopy apparatus which permits direct visualization and modeling of ultra fast, broadband energy relaxation.
It is another object of the present invention to provide an apparatus which achieves highly sensitive chirp-free transient absorption spectroscopy measurements.
It is a further object of the present invention to provide a highly sensitive, chirp-free femtosecond transient absorption spectroscopy apparatus using a Tl:sapphire laser source, wherein two or more laser signals are applied to a material to be sampled for data acquisition.
It is a further object of the present invention to provide a method and apparatus for femtosecond transient absorption comprising phase-sensitive detection, spectral scanning and simultaneous controlling of a delay translation stage resulting in highly-sensitive, chirp-free measurements of femtosecond transient absorption characteristics of a material over the entire bandwidth of a white-light or infrared continuum.
Additional objects, advantages and novel features of the invention will be set forth in part in the description that follows, and in part will become apparent to those of skill in the art upon examination of the following description or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.